Fix for #1239

This fix error message like this in case of bzlmod usage:
invalid registered toolchain '@rules_foreign_cc//toolchains:preinstalled_nmake_toolchain': No repository visible as '@rules_foreign_cc' from main repository

foreign_cc/built_tools/pkgconfig_build.bzl

@@ -166,7 +166,7 @@ def pkgconfig_tool(name, srcs, **kwargs):
             "@platforms//os:windows": "cp release/x64/pkg-config.exe $$INSTALLDIR$$/bin",
             "//conditions:default": "",
         }),
-        toolchain = "@rules_foreign_cc//toolchains:preinstalled_nmake_toolchain",
+        toolchain = str(Label("//toolchains:preinstalled_nmake_toolchain")),
         tags = tags,
         **kwargs
     )